J D DewarConsultancy32 Tolethorpe close

OakhamRutland

LE15 6GFUK

Tel/ Fax 01572 774 338e-mail joedewar@btopenworld.com

TECHNICAL NOTE 1022Guidance on the use of MixSim for the design of concrete mixtures when EN 206 is thebasis of concrete specification, production and conformity.

From 1 December 2003, the CEN standard EN 206 becomes the basis of concretespecification, production and conformity for CEN members throughout Europe. By the samedate, the existing corresponding national standards of CEN members are required to bewithdrawn.

EN 206 contains a number of important aspects permitted to be referred to provisions innational standards (which can be expected to differ between countries), e.g. limiting values fordurability parameters.

In the UK, for example, the BSI has published BS EN 206-1:2000, being EN 206 in theEnglish language, together with BS 8500-1:2002 and BS 8500-2:2002, being complementaryBritish standards detailing the UK provisions. The current British Standard BS 5328 will bewithdrawn.

The design of concrete mixtures is a key part of concrete production and, while EN 206 exertsrelatively little influence directly on the design process, it does of course have considerableindirect effect through the requirements for specification and conformity in EN 206 togetherwith the permitted national variations.

Fortunately, the use of MixSim for design is not greatly affected by the adoption of EN206because most of the various options covered by EN 206 had been incorporated already intoMixSim to cater for the very wide range of options needed up to now for specificationsworldwide.

Generally.

It has been traditional in different countries for the specified values of some parameters to beabsolute minima or maxima, characteristic values, target mean values, tolerances or ranges.EN 206 similarly adopts different approaches for different parameters and in some casesallows alternative approaches for the same parameter to be used for specification purposes.

MixSim adopts the characteristic value approach for strength, the target mean for slump andfor air, minimum target mean for cement content and maximum target mean for water/cementratio. The user of MixSim needs to decide whether to allow additionally for variation takingaccount of the way in which each parameter has been specified and the consequences offailing to conform.

With regard to the properties of materials, it is always necessary for users of MixSim to ensurethat materials selected to be included in designs are permitted by the specifications and thatany restrictions on properties of the materials or on concrete made with them are taken intoaccount.

For the use together of EN206 and MixSim there are just a few aspects requiring morespecific guidance.

Aggregates

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DRAFTThe new series of standard sieve sizes for aggregates for concrete are shown in Table 1

Table 1 Sieve series for coarse and fine aggregates for concrete

Coarse aggregatesieve series (mm)

Fine aggregatesieve series (mm)

80 463 2.840 2

31.5 120 0.516 0.2514 0.12510 0.0638

6.34

The significance for MixSim relates only to the sieve sets used to calculate the mean sizes ofthe materials. To modify any of the sets it is necessary only to select the Sieve Sets button onthe Materials Database screen and then edit the sieve sizes as necessary. It is recommendedto include some sieves outside the nominal limits to allow for undersize and oversize material.

The present MixSim sieve sets (assuming the user has not already made changes) andrecommended sets for future use are shown in Figure 1

NOTE. Changing the sieve sets for future use will not affect the sieve sets adoptedpreviously for existing materials in the database.

Figure 1 The dropdown screen for the sieve sets used for calculating meansizes of materials, before and after modification to the right-hand pair of columns toallow for European standardisation of aggregate test sieves

The designations for aggregates have also changed, thus for the UK some examples areshown in .

Table 2 Some examples of aggregate designations by size

Existing designations New designations20 mm single size 10/2010 mm single size 4/10

20-5 graded 4/20M (medium) sand 0/4 or 0/2

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The new designations are established from the nominal lower and upper sieve sizes for theparticular aggregates, the lower size being stated first. The effect for MixSim relates only tothe descriptions of the aggregates in the Materials Database.

Additions

EN 206 caters for the use of Type 1 (nearly inert) additions and Type 2 (pozzolanic or latenthydraulic) additions.

MixSim allows already for their use taking account of their effects on water demand andstrength and on certain restrictions placed upon their use in specifications such as EN 206for durability, e.g. the proportion (k) of addition in the combination with cement permitted tocount towards satisfying specified limits for minimum cement content and maximumwater/cement ratio.

EN 206 allows certain additions to be used with corresponding stated maximum k values fordurability and also allows national rules to be applied to these and other additions. Forexample, in the UK, some additions may count fully towards durability provided special tests ofthe combinations have been made.

MixSim also allows for use of the efficiency (strength factor or k value) of the addition relativeto the cement with respect to strength.

Specified values relating to durability are usually safe absolute maxima whereas the k valuesfor strength design should be target mean values determined for specific combinations ofadditions and cements.

For MixSim, the specified maximum values for durability, expressed as percentages, shouldbe entered in the “percentages permitted to count as cement” whereas for strength the targetmean values should be entered in the conditions section, in decimal format, against “strengthfactors” for additions.

NOTE. It should also be noted that for strength, factors usually vary with age at test. Atpresent MixSim permits only a single value of strength factor to be entered for each material.

MixSim does not yet include a calculator enabling strength factor for an addition to becalculated from concrete trial data. However, such a calculator, in MS Excel spreadsheetformat, may be obtainable from the supplier of MixSim.

Design for workability of fresh concrete

MixSim design is based on selection of a target slump. EN 206 permits specification of targetslump but also permits specification by slump class and by other test methods than slump.

The EN 206 slump classes are shown in Table 3 together with recommendations forcorresponding design slumps for use in MixSim.

NOTE. The suggested design slump in the final column is lower than the arithmetic meanvalue of the range in the middle column to allow for the non-linear relation between slump andwater content.

Table 3 EN 206 slump classes

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Where another workability method than slump is specified it is recommended that arelationship between the methods is established so that the corresponding slump may beused in MixSim.

Where the corresponding slump is expected to be outside the normal working range forslump, e.g. below zero slump or above 200 mm slump, it is recommended to make a trialconcrete batch, measure the water content at a reference slump, (within the normal workingrange for slump), and the water content for the chosen workability by the specified method (orto suit the method of compaction) and to calculate the water factor and corresponding designslump using the following formulae.

Water factor 'Water for spec work yWF

Water for reference slump

Correponding design slump6 57 6WFDesign slump ref slumpWF

Example Reference slump 50 mm

Water for specified or target workability 140 kg/m3

Water for 50mm slump 180 kg/m3

Water factor WF = 140/180 = 0.78

Design slump DS = 50 x ( 6 x 0.78 – 5 )/ ( 7 – 6x 0.78 )

= 50x (0.32)/ (2.32)

= 7 mm

NOTE. In this case the design slump is negative but can be used successfully withinMixSim to design concrete having a lower workability than zero slump, by ensuringthe negative sign is included.

Design for air content of fresh concrete.

For non-air entrained concrete, air content is not normally specified but entrapped air alwaysneeds to be allowed for in design. MixSim allows the user to input target values for total andentrapped air and this should still be done for EN 206 concrete.

For air entrained concrete, EN 206 specifies minimum total air content and a maximum totalair content 4% higher than the specified minimum. Thus, a target mean value of specifiedminimum plus 2% is implied.

MixSim currently requires input of the target mean value for air, so that for EN 206 thespecified minimum air content plus 2% should be entered against total air. The expectedmean content of entrapped air (say 0.5 or 1%) included within this total air content should stillbe entered because the two types of air (entrained and entrapped) in air entrained concrete

EN206 EN 206 Suggestedslump slump targetclass mm slump mm

S1 10-40 20S2 50-90 65S3 100-150 120S4 160-210 180

DRAFTaffect strength differently. The difference between the two entered values is the entrained aircontent which is not specified directly in EN 206 but is shown for information in the MixSimentries.

Design for minimum cement content and maximum water/cement ratio.

EN206 requires specification of minimum cement content and maximum water/cement ratio.For conformity, the producer is required to demonstrate from batch records achievement of atleast the specified minimum cement content less 10 kg and not more than the specifiedmaximum water/cement ratio plus 0.02.

MixSim currently allows for input of both the specified minimum cement content and themaximum water/cement ratio as limiting target mean values. Thus, the user needs to considerwhether, on the basis of past data, it is necessary to include design margins in the inputvalues to take account of the conformity requirements of EN 206.

Design for compressive strength. EN 206 classifies strength in terms of 28 day characteristic strengths on the basis of cylindersand cubes, e.g. C25/30, where the first number is the strength of a 150 mm dia, 300 mm highcylinder and the second number is the 150 mm cube strength.

NOTE. The EN206 dual numbering system may be confused with some well establishedsystems which combined the strength with some other feature of the specification, e.gmaximum aggregate size. Also , it should not be assumed that by giving both cube andcylinder strength a particular relationship is being assumed for purposes of conversion formixture design or control. The user will need to establish relationships for different materialsand strength levels if it becomes necessary to work with both systems.

MixSim enables the user to adopt either cubes or cylinders as the basis for strength using theoption located under the utilities menu. Once adopted, the selection must not be changed andthe corresponding strength basis must be maintained for all input data for specified strength,standard deviation and concrete trial strength results.

NOTE. The strength factors calculator Method 2 operates currently on mortar prisms forcement strength and cubes not cylinders for concrete strength. Method 1 operates on cubestrengths. This may be the subject of review for a later version of MixSim.

When the user adopts the utilities option for cylinders, the formulae generated for strength vw/c are adjusted automatically but approximately to allow for the lower strengths comparedwith concrete cubes.

If the user has data available on cylinder strength v w/c enabling use of method 1 in thestrength factors calculator, then the factors displayed for cement strength and for aggregatewill also include a factor for cube to cylinder strength conversion. It will then not be necessaryto use the utilities option for cylinders and, provided the specified strength, standard deviationand trials data are all entered as cylinder strengths, MixSim will provide designs and data interms of cylinder strength.

MixSim allows the user to select a strength margin factor and the standard deviation forcalculation of the target mean to suit the strength specified and the degree of safety requiredto take account of the conformity rules for strength and for production control in EN 206.Whether these rules enable a relaxation or a tightening of the margins for use in MixSim willdepend on detailed examination of EN206 by the user taking into account current companyand national practices, and the rules of quality certification schemes.

NOTE. The appropriate margin factor and standard deviation for cylinders may well differ fromthose for cubes.

Design for tensile strength

MixSim provides no direct method for design for tensile strength but MixSim can be used onthe basis of compressive strength by first determining the relation between tensile andcompressive strength from concrete trials . The relation can be expected to be materialsensitive.

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References and further reading

BSI BS EN 206-1:2000 Concrete- Part 1: Specification, performance, production and conformity.

BS 8500 Concrete- Complementary British Standard to BS EN 206- 1

Part 1: 2002 Method of specifying and guidance for the specifier.Part 2: 2002 Specification for constituent materials and concrete.

Harrison, TA. The new concrete standards- getting started. QPA, 2003

QPA Aggregates- EN day is fast approaching. Aggregates Group Bulletin 1.Aggregates for concrete- Implementation day 1 January 2004. Aggregates GroupBulletin 2.

Dr J D Dewar, J D Dewar Consultancy Draft 10 October 2003 Fig 1 amended, Draft Rev1 12 October 2004